Borohydride compounds have a high hydrogen storage rate and can easily generate hydrogen by reacting with water under normal temperature and normal pressure and therefore attract attention as hydrogen carriers for fuel cells. For example, in the case of sodium borohydride (NaBH4), 10.8% by mass of hydrogen can be removed by the hydrolysis reaction represented by the following chemical formula 1. In this manner, sodium borohydride containing hydrogen at high density can be an excellent hydrogen carrier.NaBH4+2H2O→4H2+NaBO2  [Chem. 1]
Methods for industrially producing sodium borohydride include a method of heating sodium tetraborate (Na2B4O7) obtained by purifying borax, metal sodium, and silicon oxide to 400 to 450° C. under a pressurized hydrogen atmosphere to allow the reaction represented by the following chemical formula 2 to proceed. However, a problem of this method is that the cost of the purification of borax that is a raw material is high, and expensive metal sodium is used, and therefore the production cost increases.Na2B4O7+16Na+8H2+7SiO2→4NaBH4+7Na2SiO3  [Chem. 2]
In addition, it is reported that sodium borohydride can also be produced by reacting sodium diborate (Na4B2O5) or a mixture of sodium diborate, sodium metaborate (NaBO2), and sodium oxide (Na2O) with aluminum (Al) and hydrogen (H2) as shown in the following chemical formula 3 and chemical formula 4 (see Non Patent Literature 1). In the reactions represented by the following chemical formulas 3 and 4, aluminum more inexpensive than sodium is used, but Na4B2O5 and NaBO2 that are raw materials are expensive, and therefore, as in the above-described method, a problem is that the production cost increases.4Al+6H2+2Na4B2O5→3NaBH4+4NaAlO2+NaBO2  [Chem. 3]4Al+6H2+Na4B2O5+NaBO2+Na2O→3NaBH4+4NaAlO2  [Chem. 4]
Therefore, conventionally, a method for producing a borohydride compound by the reaction represented by the following chemical formula 5 using sodium metaborate (NaBO2) produced by hydrolysis and a metal hydride such as magnesium hydride (MgH2) is proposed (for example, see Patent Literature 1).NaBO2+2MgH2→NaBH4+2MgO  [Chem. 5]
In addition, a method of heating a mixture comprising a borate and an alkaline earth metal such as magnesium to, for example, 550° C., under a pressurized hydrogen atmosphere to obtain sodium borohydride by the reaction represented by the following chemical formula 6 is also proposed (for example, see Patent Literature 2). In the method described in Patent Literature 2, by performing heating at a pressure lower than reaction equilibrium pressure at which a hydride of magnesium is stably present to produce protide (H−) on the surface of magnesium, an exchange reaction with an oxide ion (O2−) in sodium metaborate is promoted for yield improvement.NaBO2+2Mg+2H2→NaBH4+2MgO  [Chem. 6]
In the methods described in Patent Literatures 1 and 2 described above, the reaction is performed using a batch type apparatus, but an apparatus that continuously performs the reaction represented by the above chemical formula 3 is also proposed (see Patent Literature 3). The apparatus described in Patent Literature 3 has a configuration in which a mixture comprising a borate and an alkaline earth metal is supplied into a heated cylinder together with pressurized hydrogen gas, and these are kneaded by a screw blade while being transferred, to allow a reaction to proceed to produce a tetrahydroborate.